Duplex stainless steel and method for producing duplex stainless steel

ABSTRACT

A duplex stainless steel with occurrence of pitting suppressed is provided. A duplex stainless steel according to the present disclosure has a chemical composition consisting of, in mass %, Cr: more than 27.00% to 29.00%, Mo: 2.50 to 3.50%, Ni: 5.00 to 8.00%, W: 4.00 to 6.00%, Cu: 0.01 to less than 0.10%, N: more than 0.400% to 0.600%, C: 0.030% or less, Si: 1.00% or less, Mn 1.00% or less, sol.Al: 0.040% or less, V: 0.50% or less, O: 0.010% or less, P: 0.030% or less, and S: 0.020% or less with the balance being Fe and impurities and satisfying Formula (1), a microstructure consisting of 35 to 65 volume % of ferrite phase with the balance being the austenite phase, and the area fraction of Cu precipitated in the ferrite phase is 0.5% or less. 
       Cr+4.0×Mo+2.0×W+20×N−5×ln(Cu)≥65.2  (1)

TECHNICAL FIELD

The present invention relates to a duplex stainless steel and a methodfor producing the duplex stainless steel.

BACKGROUND ART

A duplex stainless steel having a dual phase structure consisting of theferrite phase and the austenite phase is known to have excellentcorrosion resistance. A duplex stainless steel is particularly superiorin corrosion resistance against pitting and/or crevice corrosion(hereinafter referred to as “pitting resistance”), which is taken as aproblem in an aqueous solution containing chlorides. A duplex stainlesssteel is therefore widely used in a wet environment containingchlorides, such as seawater. In a wet environment containing chlorides,a duplex stainless steel is used, for example, in a flow line pipe, anumbilical tube, and a heat exchanger.

In recent years, the corrosion conditions in the environment in which aduplex stainless steel is used have been increasingly severe. A duplexstainless steel is therefore required to have more excellent pittingresistance. To further enhance the pitting resistance of a duplexstainless steel, a variety of technologies have been proposed.

International Application Publication No. 2013/191208 (PatentLiterature 1) discloses a duplex stainless steel containing, in mass %,Ni: 3 to 8%, Cr: 20 to 35%, Mo: 0.01 to 4.0%, and N: 0.05 to 0.60% andfurther containing one or more types of element selected from Re: 2.0%or less. Ga: 2.0% or less, and Ge: 2.0% or less. In Patent Literature 1,the fact that the duplex stainless steel contains Re, Ga, or Geincreases the critical potential at which pitting occurs (pittingpotential) to enhance the pitting resistance and crevice corrosionresistance.

International Application Publication No. 2010/082395 (Patent Literature2) discloses a method for producing a duplex stainless steel pipe byperforming hot working or hot working and further solid solution heattreatment on a duplex stainless steel material containing in mass %, Cr:20 to 35%, Ni: 3 to 10%, Mo: 0 to 6%, W: 0 to 6%, Cu: 0 to 3%, and N:0.15 to 0.60% to produce a steel pipe for cold working and thenperforming cold rolling on the steel pipe. The method for producing aduplex stainless steel pipe in Patent Literature 2 is a method forproducing a duplex stainless steel pipe having a minimum yield strengthranging from 758.3 to 965.2 MPa by performing cold rolling that allowsthe working ratio Rd(=exp[{In(MYS)−In(14.5×Cr+48.3×Mo+20.7×W+6.9×N)}/0.195]) at the areareduction ratio in the final cold rolling step to fall within a rangefrom 10 to 80%. Patent Literature 2 describes that the method describedabove provides a duplex stainless steel pipe that can be used, forexample, in an oil well and a gas well, shows excellent corrosionresistance also in a carbon dioxide gas corrosion environment or astress corrosion environment, and has high strength.

Japanese Patent Application Publication No. 2007-84837 (PatentLiterature 3) discloses a duplex stainless steel containing, in mass %,Cr: 20 to 30%, Ni: 1 to 11%. Cu: 0.05 to 3.0%, Nd: 0.005 to 0.5%, and N:0.1 to 0.5% and/or Mo: 0.5 to 6% and W: 1 to 10%. In Patent Literature3, the hot workability of the duplex stainless steel is enhanced becausethe duplex stainless steel contains Nd.

National Publication of International Patent Application No. 2005-520934(Patent Literature 4) discloses a super duplex stainless steelcontaining, in weight %, Cr: 21.0% to 38.0%, Ni: 3.0% to 12.0%, Mo: 1.5%to 6.5%, W: 0 to 6.5%, N: 0.2% to 0.7%, and Ba: 0.0001 to 0.6% andhaving a pitting resistance equivalent index PREW that satisfies40≤PREWs≤67. Patent Literature 4 describes that the thus configuredsuper duplex stainless steel is superior in corrosion resistance,embrittlement resistance, castability, and hot workability withformation of intermetal phases, such as the brittle sigma (σ) phase andthe chi (χ) phase, suppressed.

CITATION LIST Patent Literature

Patent Literature 1: International Application Publication No.2013/191208

Patent Literature 2: International Application Publication No.2010/082395

Patent Literature 3: Japanese Patent Application Publication No.2007-84837

Patent Literature 4: National Publication of International PatentApplication No. 2005-520934

SUMMARY OF INVENTION Technical Problem

As described above, a duplex stainless steel having more excellentpitting resistance has been required in recent years. Technical meansother than the technologies described in Patent Literatures to 4 maytherefore provide a duplex stainless steel showing excellent pittingresistance.

An objective of the present disclosure is to provide a duplex stainlesssteel having excellent pitting resistance and a method for producing theduplex stainless steel.

Solution to Problem

A duplex stainless steel according to the present disclosure has achemical composition consisting of, in mass %, Cr: more than 27.00% to29.00%, Mo: 2.50 to 3.50%, Ni: 5.00 to 8.00%, W: 4.00 to 6.00%, Cu: 0.01to less than 0.10%, N: more than 0.400% to 0.600%, C: 0.030% or less,Si: 1.00% or less, Mn: 1.00% or less, sol.Al: 0.040% or less, V: 0.50%or less. O: 0.010% or less, P: 0.030% or less, S: 0.020% or less, Ca: 0to 0.0040%, Mg: 0 to 0.0040%, and B: 0 to 0.0040% with the balance beingFe and impurities and satisfying Formula (1), and a microstructureconsisting of 35 to 65 volume % of ferrite phase with the balance beingan austenite phase. In the duplex stainless steel according to thepresent disclosure, an area fraction of Cu precipitated in the ferritephase is 0.5% or less.

Cr+4.0×Mo+2.0×W+20×N−5×ln(Cu)≥65.2  (1)

where, a content in mass % of each of the elements is substituted into acorresponding symbol of the element in Formula (1).

A method for producing a duplex stainless steel according to the presentdisclosure includes a preparation step, a hot working step, a coolingstep, and a solution heat treatment step. In the preparation step, astarting material having the chemical composition described above isprepared. In the hot working step, the starting material is subjected tohot working at 850° C. or more. In the cooling step, the startingmaterial subjected to the hot working is cooled at a rate of 5° C./secor more. In the solution heat treatment step, the cooled startingmaterial is subjected to a solution heat treatment at 1070° C. or more.

Advantageous Effects of Invention

The duplex stainless steel according to the present disclosure hasexcellent pitting resistance. The method for producing the duplexstainless steel according to the present disclosure allows production ofthe duplex stainless steel described above.

DESCRIPTION OF EMBODIMENTS

The present inventors have investigated and studied an approach forenhancing the pitting resistance of a duplex stainless steel. As aresult, the following findings have been achieved.

Cr, Mo, and Cu are known to be effective in improvement of the pittingresistance of a duplex stainless steel. Among Cr, Mo, and Cu, Cr and Moare believed to have a mechanism that enhances the pitting resistance ofa duplex stainless steel as follows: Cr serves as a primary component ofa passive film as an oxide on the surface of a duplex stainless steel.The passive film prevents contact between corrosion factors and thesurface of the duplex stainless steel. As a result, the duplex stainlesssteel on the surface of which the passive film has been formed hasenhanced pitting resistance. Mo is contained in the passive film andfurther enhances the pitting resistance of the passive film.

On the other hand, among Cr, Mo, and Cu, Cu is believed to have amechanism that enhances the pitting resistance of a duplex stainlesssteel as follows: It is believed that there are the following two stepsthat cause pitting to occur. The first step is occurrence of pitting(initial stage). The next step is propagation of the pitting(propagation stage). It has been believed that Cu is effective insuppressing the propagation of pitting. Particularly in an acidicsolution, an active site where the duplex stainless steel melts at highspeed is formed on the surface of the duplex stainless steel. Cu coatsthe active site to suppress the melting of the duplex stainless steel.It has been believed that the thus functioning Cu suppresses thepropagation of the pitting that occurs on a duplex stainless steel.

It has been believed based on the mechanism described above that Cr, Mo,and Cu are elements effective in improvement in pitting resistance of aduplex stainless steel. Cr, Mo, and Cu have therefore been activelycontained in a duplex stainless steel to enhance the pitting resistance.However, the following findings that had not been known have beenobtained as a result of the studies conducted by the present inventors.Specifically, the present inventors have found that among Cr, Mo, andCu, Cu instead lowers the pitting resistance in some cases at theoccurrence of pitting (initial stage).

Table 1 is a table showing the chemical compositions of test specimenslabeled with test numbers 2 and 5 and the pitting potential, which is anindex of the pitting resistance, of the test specimens in Examplesdescribed later. The chemical compositions listed in two rows in Table 1are those of steels of B and E, correspond to the test numbers 2 and 5,and are extracted from Table 3, which will be described later. Thechemical compositions in Table 1 are expressed in mass %, and thebalance is Fe and impurities. The pitting potentials listed in Table 1are those labeled with the corresponding test numbers and are extractedfrom Table 4, which will be described later.

TABLE 1 Test No. Steel Cr Mo Ni W Cu N C Si Mn 2 B 28.10 3.11 5.31 4.190.14 0.421 0.016 0.49 0.97 5 E 27.53 2.61 6.97 4.31 0.04 0.419 0.0160.48 0.92 Test No. Steel sol. Al V O P S Ca Mg B 2 B 0.013 0.10 0.0040.018 0.001 0.0025 0.0001 0.0019 5 E 0.017 0.10 0.005 0.016 0.001 0.00100.0025 0.0013 Test Pitting potential No. Steel (mVvs · SCE) 2 B 71 5 E346

Referring to Table 1, the test specimen labeled with the test number 2has a higher Cu content than the Cu content in the test specimen labeledwith the test number 5. Further, the test specimen labeled with the testnumber 2 has higher Cr and Mo contents than the Cr and Mo contents inthe test specimen labeled with the test number 5. It can therefore beexpected based on the findings in the related art that the test specimenlabeled with the test number 2, which has higher Cr, Mo, and Cucontents, has more excellent pitting resistance than the test specimenlabeled with the test number 5. The pitting potential, which is an indexof the pitting resistance, of the test specimen labeled with the testnumber 2 is, however, 71 mVvs.SCE, which is smaller than the pittingpotential of 346 mVvs.SCE of the test specimen labeled with the testnumber 5.

That is, the pitting resistance of the test specimen labeled with thetest number 2, which is expected based on the findings in the relatedart to have more excellent pitting resistance than the test specimenlabeled with the test number 5, is instead smaller than the pittingresistance of the test specimen labeled with the test number 5. In viewof the fact described above, the present inventors have focused on themicrostructures of the test specimens labeled with the test numbers 2and 5 and have investigated the microstructures in more detail. As aresult, the investigation clearly showed that the test specimen labeledwith the test number 2 has a greater area fraction of Cu precipitated inthe ferrite phase (called Cu area faction in ferrite phase) than thetest specimen labeled with the test number 5.

In view of the fact described above, the present inventors haveinvestigated and studied the effect of Cu precipitated in the ferritephase on the pitting resistance of the duplex stainless steel. Table 2is a table showing the chemical compositions of test specimens labeledwith the test numbers 3 and 6, the Cu area fractions thereof in theferrite phase, and the pitting potential thereof which is an index ofthe pitting resistance, in Examples described later. The chemicalcompositions listed in two rows in Table 2 are those of steel of C,correspond to the test numbers 3 and 6, and are extracted from Table 3,which will be described later. The chemical compositions in Table 2 areexpressed in mass %, and the balance is Fe and impurities. The Cu areafractions thereof in the ferrite phase listed in Table 2 are thoselabeled with the corresponding test numbers and are extracted from Table4, which will be described later. The pitting potentials listed in Table2 are those labeled with the corresponding test numbers and areextracted from Table 4, which will be described later.

TABLE 2 Test No. Steel Cr Mo Ni W Cu N C Si Mn 3 C 28.24 2.96 5.76 4.250.08 0.416 0.014 0.51 0.91 6 C 28.24 2.96 5.76 4.25 0.08 0.416 0.0140.51 0.91 Test No. Steel sol. Al V O P S Ca Mg B 3 C 0.012 0.10 0.0040.019 0.001 0.0015 0.0002 0.0012 6 C 0.012 0.10 0.004 0.019 0.001 0.00150.0002 0.0012 Test Cu area fraction Pitting potential No. Steel inferrite phase (%) (mVvs · SCE) 3 C 0.7 −12 6 C 0 204

Referring to Table 2, the test specimen labeled with the test number 3and the test specimen labeled with the test number 6 had the samechemical composition. On the other hand, the test specimen labeled withthe test number 6 had a smaller Cu area fraction in the ferrite phasethan the Cu area fraction in the ferrite phase of the test specimenlabeled with the test number 3. As a result, the pitting potential ofthe test specimen labeled with the test number 6 was 204 mVvs.SCE, whichwas greater than the pitting potential of −12 mVvs.SCE of the testspecimen labeled with the test number 3. That is, the test specimenlabeled with the test number 6 had more excellent pitting resistancethan the test specimen labeled with the test number 3 as a result of adecrease in the amount of precipitation of Cu in the ferrite phase inthe test specimen labeled with the test number 6.

It has been believed as described above that increasing the Cr, Mo, andCu contents increases the pitting resistance. The present inventorshave, however, found for the first time that Cu among Cr, Mo, and Cu isinstead likely to lower the pitting resistance. The present inventorshave further found that reduction in the amount of Cu precipitating inthe ferrite phase allows enhancement of the pitting resistance, which isa finding that has not been known at all.

No detailed reason why Cu precipitated in the ferrite phase lowers thepitting resistance of a duplex stainless steel has been clarified. Thepresent inventors, however, consider the reason as follows: Cuprecipitated in the ferrite phase is likely to prevent uniform formationof a passive film. Therefore, in a case where a large amount of Cu hasprecipitated in the ferrite phase, the large amount of Cu is likely tolower the passive film's effect of suppressing the contact betweencorrosion factors and the surface of the duplex stainless steel. Thepresent inventors believe that pitting occurs on the surface of theduplex stainless steel as a result of the assumption described above.

A duplex stainless steel according to the present embodiment attainedbased on the findings described above has a chemical compositionconsisting of in mass %. Cr: more than 27.00% to 29.00%, Mo: 2.50 to3.50%. Ni: 5.00 to 8.00%, W: 4.00 to 6.00%, Cu: 0.01 to less than 0.10%,N: more than 0.400% to 0.600%, C: 0.030% or less, Si: 1.00% or less. Mn:1.00% or less, sol.Al: 0.040%, or less. V: 0.50% or less, O: 0.010% orless, P: 0.030% or less, S: 0.020% or less, Ca: 0 to 0.0040%, Mg: 0 to0.0040%, and B: 0 to 0.0040% with the balance being Fe and impuritiesand satisfying Formula (1), and a microstructure consisting of 35 to 65volume % of ferrite phase with the balance being the austenite phase. Inthe duplex stainless steel according to the present embodiment, the areafraction of Cu precipitated in the ferrite phase is 0.5% or less.

Cr+4.0×Mo+2.0×W+20×N−5×ln(Cu)≥65.2  (1)

where, the content in mass % of each of the elements is substituted intothe corresponding symbol of the element in Formula (1).

The duplex stainless steel according to the present embodiment has thechemical composition described above and the microstructure describedabove, and the area fraction of Cu in the ferrite phase is 0.5% or less.As a result, the duplex stainless steel according to the presentembodiment has excellent pitting resistance.

The chemical composition described above preferably contains, in mass %,one or more types of element selected from the group consisting of Ca:0.0001 to 0.0040%, Mg: 0.0001 to 0.0040%, and B: 0.0001 to 0.0040%.

In this case, the duplex stainless steel according to the presentembodiment has enhanced hot workability.

A method for producing a duplex stainless steel according to the presentembodiment includes a preparation step, a hot working step, a coolingstep, and a solution heat treatment step. In the preparation step, astarting material having the chemical composition described above isprepared. In the hot working step, the starting material is subjected tohot working at 850° C. or more. In the cooling step, the startingmaterial subjected to the hot working is cooled at a rate of 5° C./secor more. In the solution heat treatment step, the cooled startingmaterial is subjected to a solution heat treatment at 1070° C. or more.

The duplex stainless steel produced by the production method accordingto the present embodiment has the chemical composition described aboveand the microstructure described above, and the area fraction of Cu inthe ferrite phase is 0.5% or less. As a result, the duplex stainlesssteel produced by the production method according to the presentembodiment has excellent pitting resistance.

The duplex stainless steel according to the present embodiment will bedescribed below in detail.

[Chemical Composition]

The chemical composition of the duplex stainless steel according to thepresent embodiment contains the following elements. The symbol %associated with an element means mass % unless otherwise specified.

[Essential Elements]

The chemical composition of the duplex stainless steel according to thepresent embodiment essentially contains the following elements:

Cr: more than 27.00% to 29.00%

Chromium (Cr) forms a passive film as an oxide on the surface of theduplex stainless steel. The passive film prevents contact betweencorrosion factors and the surface of the duplex stainless steel. As aresult, occurrence of pitting on the duplex stainless steel issuppressed. Further, Cr is an element necessary for achievement of theferrite structure in the duplex stainless steel. Achievement of asufficient ferrite structure provides stable pitting resistance. Too lowa Cr content provides no effects described above. On the other hand, toohigh a Cr content lowers the hot workability of the duplex stainlesssteel. The Cr content therefore ranges from more than 27.00% to 29.00%.The lower limit of the Cr content is preferably 27.50%, more preferably28.00%. The upper limit of the Cr content is preferably 28.50%.

Mo: 2.50 to 3.50%

Molybdenum (Mo) is contained in the passive film and further enhancesthe corrosion resistance of the passive film. As a result, the pittingresistance of the duplex stainless steel is enhanced. Too low a Mocontent provides no effect described above. On the other hand, too higha Mo content lowers the workability of, for example, the assembly of asteel pipe made of the duplex stainless steel. The Mo content thereforeranges from 2.50 to 3.50%. The lower limit of the Mo content ispreferably 2.80%, more preferably 3.00%. The upper limit of the Mocontent is preferably 3.30%.

Ni: 5.00 to 8.00%

Nickel (Ni) is an austenite stabilizing element and is an elementnecessary for achievement of the ferrite/austenite dual phase structure.Too low a Ni content provides no effect described above. On the otherhand, too high a Ni content causes imbalance between the ferrite phaseand the austenite phase. In this case, the duplex stainless steel is notstably produced. The Ni content therefore ranges from 5.00 to 8.00%. Thelower limit of the Ni content is preferably 5.50%, more preferably6.00%. The upper limit of the Ni content is preferably 7.50%.

W: 4.00 to 6.00%

Tungsten (V) is contained in the passive film and further enhances thecorrosion resistance of the passive film, as in the case of Mo. As aresult, occurrence of the pitting on the duplex stainless steel issuppressed. Too low a W content provides no effect described above. Onthe other hand, too high a W content is likely to cause the a phase toprecipitate easily, resulting in a decrease in toughness. The W contenttherefore ranges from 4.00 to 6.00%. The lower limit of the W content ispreferably 4.50%. The upper limit of the W content is preferably 5.50%.

Cu: 0.01 to less than 0.10%

Copper (Cu) is an element effective in suppressing the propagation ofthe pitting (propagation stage). Too low a Cu content provides no effectdescribed above. On the other hand, among Cr, Mo, and Cu, Cu lowers thepitting resistance at the occurrence of pitting (initial stage). Theduplex stainless steel according to the present embodiment therefore hasa lowered Cu content as compared with the Cu content in a duplexstainless steel of the related art. As a result, the precipitation of Cuin the ferrite phase is suppressed, and occurrence of pitting on theduplex stainless steel (initial stage) is suppressed. Too high a Cucontent causes too large an area fraction of Cu in the ferrite phase. Inthis case, the pitting resistance of the duplex stainless steel lowers.The Cu content therefore ranges from 0.01 to less than 0.10%. The upperlimit of the Cu content is preferably 0.07%, more preferably 0.05%.

N: more than 0.400% to 0.600%

Nitrogen (N) is an austenite stabilizing element and is an elementnecessary for achievement of the ferrite/austenite dual phase structure.N further enhances the pitting resistance of the duplex stainless steel.Too low a N content provides no effects described above. On the otherhand, too high a N content lowers the toughness and the hot workabilityof the duplex stainless steel. The N content therefore ranges from morethan 0.400% to 0.600%. The lower limit of the N content is preferably0.420%. The upper limit of the N content is preferably 0.500%.

C: 0.030% or less

Carbon (C) is inevitably contained. That is, the C content is more than0%. C forms a Cr carbide in the crystal grain boundary, and the Crcarbide increases the corrosion susceptibility in the grain boundary.The C content is therefore 0.030% or less. The upper limit of the Ccontent is preferably 0.025%, more preferably 0.020%. The C content ispreferably minimized. Extreme reduction in the C content, however,greatly increases the production cost. The lower limit of the C contentis therefore preferably 0.001%, and more preferably 0.005% inconsideration of industrial production.

Si: 1.00% or less

Silicon (Si) deoxidizes steel. In a case where Si is used as adeoxidizer, the Si content is more than 0%. On the other hand, too higha Si content lowers the hot workability of the duplex stainless steel.The Si content is therefore 1.00% or less. The upper limit of the Sicontent is preferably 0.80%, and more preferably 0.70%. The lower limitof the Si content is not limited to a specific value and is, forexample, 0.20%.

Mn: 1.00% or less

Manganese (Mn) deoxidizes steel. In a case where Mn is used as adeoxidizer, the Mn content is more than 0%. On the other hand, too higha Mn content lowers the hot workability of the duplex stainless steel.The Mn content is therefore 1.00% or less. The upper limit of the Mncontent is preferably 0.80%, and more preferably 0.70%. The lower limitof the Mn content is not limited to a specific value and is, forexample, 0.20%.

Sol. Al: 0.040% or less

Aluminum (Al) deoxidizes steel. In a case where Al is used as adeoxidizer, the Al content is more than 0%. On the other hand, too highan Al content lowers the hot workability of the duplex stainless steel.The Al content is therefore 0.040% or less. The upper limit of the Alcontent is preferably 0.030%, and more preferably 0.025%. The lowerlimit of the Al content is not limited to a specific value and is, forexample, 0.005%. In the present embodiment, the Al content refers to theacid-soluble Al (sol.Al) content.

V: 0.50% or less

Vanadium (V) is inevitably contained. That is, the V content is morethan 0%. Too high a V content excessively increases the amount of theferrite phase, resulting in decreases in toughness and corrosionresistance of the duplex stainless steel in some cases. The V content istherefore 0.50% or less. The upper limit of the V content is preferably0.40%, and more preferably 0.30%. The lower limit of the V content isnot limited to a specific value and is, for example, 0.05%.

O: 0.010% or less

Oxygen (O) is an impurity. That is, the O content is more than 0%. Olowers the hot workability of the duplex stainless steel. The O contentis therefore 0.010% or less. The upper limit of the O content ispreferably 0.007%, and more preferably 0.005%. The O content ispreferably minimized. Extreme reduction in the O content, however,greatly increases the production cost. The lower limit of the O contentis therefore preferably 0.0001%, and more preferably 0.0005% inconsideration of industrial production.

P: 0.030% or less

Phosphorus (P) is an impurity. That is, the P content is more than 0%. Plowers the pitting resistance and toughness of the duplex stainlesssteel. The P content is therefore 0.030% or less. The upper limit of theP content is preferably 0.025%, and more preferably 0.020%. The Pcontent is preferably minimized. Extreme reduction in the P content,however, greatly increases the production cost. The lower limit of the Pcontent is therefore preferably 0.001%, and more preferably 0.005% inconsideration of industrial production.

S: 0.020% or less

Sulfur (S) is an impurity. That is, the S content is more than 0%. Slowers the hot workability of the duplex stainless steel. The S contentis therefore 0.020% or less. The upper limit of the S content ispreferably 0.010%, more preferably 0.005%, and still more preferably0.003%. The S content is preferably minimized. Extreme reduction in theS content, however, greatly increases the production cost. The lowerlimit of the S content is therefore preferably 0.0001%, and morepreferably 0.0005% in consideration of industrial production.

The balance of the chemical composition of the duplex stainless steelaccording to the present embodiment is Fe and impurities. The impuritiesin the chemical composition mean contaminants, for example, from ore asa raw material, scraps, or the production environment in industrialproduction of the duplex stainless steel that are acceptable to theextent that the contaminants do not adversely affect the duplexstainless steel according to the present embodiment.

[Optional Elements]

The chemical composition of the duplex stainless steel according to thepresent embodiment may arbitrarily contain the following elements:

Ca: 0 to 0.0040%

Calcium (Ca) is an optional element and may not be contained. That is,the Ca content may be 0%. When contained, Ca enhances the hotworkability of the duplex stainless steel. When Ca is contained even bya trace amount, the effect described above is provided to some extent.On the other hand, too high a Ca content produces a coarse oxide, whichlowers the hot workability of the duplex stainless steel. The Ca contentis therefore 0 to 0.0040%. The lower limit of the Ca content ispreferably 0.0001%, more preferably 0.0005%, and still more preferably0.0010%. The upper limit of the Ca content is preferably 0.0030%.

Mg: 0 to 0.0040%

Magnesium (Mg) is an optional element and may not be contained. That is,the Mg content may be 0%. When contained, Mg enhances the hotworkability of the duplex stainless steel, as does Ca. When Mg iscontained even by a trace amount, the effect described above is providedto some extent. On the other hand, too high a Mg content produces acoarse oxide, which lowers the hot workability of the duplex stainlesssteel. The Mg content is therefore 0 to 0.0040%. The lower limit of theMg content is preferably 0.0001%, more preferably 0.0005%, and stillmore preferably 0.0010%. The upper limit of the Ca content is preferably0.0030%.

B: 0 to 0.0040%

Boron (B) is an optional element and may not be contained. That is, theB content may be 0%. When contained, B enhances the hot workability ofthe duplex stainless steel, as do Ca and Mg. When B is contained even bya trace amount, the effect described above is provided to some extent.On the other hand, too high a B content lowers the toughness of theduplex stainless steel. The B content is therefore 0 to 0.0040%. Thelower limit of the B content is preferably 0.0001%, more preferably0.0005%, and still more preferably 0.0010%. The upper limit of the Cacontent is preferably 0.0030%.

[Formula (1)]

The chemical composition of the duplex stainless steel according to thepresent embodiment satisfies the contents of the elements describedabove and further satisfies the following Formula (1):

Cr+4.0×Mo+2.0×W+20×N−5×ln(Cu)≥65.2  (1)

where, content in mass % of each of the elements is substituted into thecorresponding symbol of the element in Formula (1).

The following definition is made: F1=Cr+4.0×Mo+2.0×W+20×N−5×ln(Cu). F1is an index representing the pitting resistance. When F1 is less than65.2, the pitting resistance of the duplex stainless steel lowers. Thefollowing formula is therefore satisfied: F1≥65.2. The lower limit of F1is preferably 68.0, more preferably 69.0, and still more preferably70.0. The upper limit of F1 is not limited to a specific value and is,for example, 90.0.

[Microstructure]

The microstructure of the duplex stainless steel according to thepresent embodiment consists of ferrite and austenite. Specifically, themicrostructure of the duplex stainless steel according to the presentembodiment consists of 35 to 65 volume % of ferrite phase with thebalance being the austenite phase. When the volume ratio of the ferritephase (hereinafter also referred to as ferrite fraction) is less than35%, stress corrosion cracking is more likely to occur depending on theenvironment in which the duplex stainless steel is used. On the otherhand, when the volume ratio of the ferrite phase is more than 65%, thetoughness of the duplex stainless steel is more likely to lower.Therefore, the microstructure of the duplex stainless steel according tothe present embodiment consists of 35 to 65 volume % of ferrite phasewith the balance being the austenite phase.

[Method for Measuring Ferrite Fraction]

In the present embodiment, the ferrite fraction of the duplex stainlesssteel can be determined by the following method: A test specimen formicrostructure observation is collected from the duplex stainless steel.When the duplex stainless steel is used to form a steel plate, a crosssection of the steel plate that is the cross section perpendicular tothe plate width direction of the steel plate (hereinafter referred to asobservation surface) is polished. When the duplex stainless steel isused to form a steel pipe, a cross section of the steel pipe that is thecross section (observation surface) containing the axial direction andthe wall thickness direction of the steel pipe is polished. When theduplex stainless steel is used to form a steel bar or a wire rod, across section of the steel bar or the wire rod that is the cross section(observation surface) containing the axial direction of the steel bar orthe wire rod is polished. The polished observation surface is thenetched by using a liquid that is the mixture of aqua regia and glycerin.

Ten visual fields of the etched observation surface are observed underan optical microscope. The area of each of the visual fields is, forexample, 2000 μm² (at magnification of 500). In each of the visualfields, the ferrite and the other phases can be distinguished from eachother based on contrast. The ferrite is therefore identified based onthe contrast in each observation. The area fraction of the identifiedferrite is measured by using a point counting method compliant with JISG0555 (2003). The measured area fraction is assumed to be equal to thevolume fraction, which is then defined as a ferrite fraction (volume %).

[Cu Area Fraction in Ferrite Phase]

The area fraction of Cu precipitated in the ferrite phase of the duplexstainless steel according to the present embodiment is 0.5% or less. Itis believed as described above that Cu contained in the duplex stainlesssteel suppresses the propagation of the pitting on the duplex stainlesssteel. The duplex stainless steel according to the present embodimenttherefore contains Cu by an amount ranging from 0.01 to less than 0.10%.On the other hand, in the duplex stainless steel containing Cu by theamount ranging from 0.01 to less than 0.10%, metal Cu precipitates inthe ferrite phase in some cases. It has clearly been shown as describedabove that Cu precipitated in the ferrite phase lowers the passivefilm's effect of suppressing occurrence of pitting. That is, metal Cuprecipitated in the ferrite phase lowers the pitting resistance of theduplex stainless steel.

The duplex stainless steel according to the present embodiment has areduced Cu area fraction in the ferrite phase to 0.5% or less. Theoccurrence of pitting on the duplex stainless steel is thus suppressed.The Cu area fraction in the ferrite phase is preferably minimized. Theupper limit of the Cu area fraction in the ferrite phase is preferably0.3%, and more preferably 0.1%. The lower limit of the Cu area fractionin the ferrite phase is 0.0%.

[Method for Measuring Cu Area Fraction in Ferrite Phase]

In the present specification, the Cu area fraction in the ferrite phasemeans the area fraction of Cu precipitated in the ferrite phase out ofthe microstructure of the duplex stainless steel with respect to theferrite phase. In the present embodiment, the Cu area fraction in theferrite phase can be measured by the following method. A thin filmspecimen for observation under a transmission electron microscope (TEM)is prepared by an FIB-micro-sampling method. To prepare the thin filmspecimen, a focused ion beam processing apparatus (MI4050 manufacturedby Hitachi High-Tech Science Corporation) is used. A thin film specimenfor TEM observation is prepared from an arbitrary portion of the duplexstainless steel. To prepare the thin film specimen, a mesh made of Moand a carbon deposit film as a surface protection film are used.

A field emission transmission electron microscope (JEM-2100Fmanufactured by JEOL Ltd.) is used for the TEM observation. The TEMobservation is performed at an observation magnification of 10000. Theferrite phase and the austenite phase in a visual field differ from eachother in terms of contrast. The crystal grain boundary is thenidentified based on the contrast. The phase of a region surrounded byeach crystal grain boundary is identified by X-ray diffraction (XRD).Among the regions surrounded by the crystal grain boundaries, the areaof the region identified as the ferrite phase is determined by imageanalysis.

Element analysis based on energy dispersive X-ray spectrometry (EDS) isperformed on the visual field under observation to generate an elementmap. Further, a precipitate can be identified based on the contrast.Therefore, whether a precipitate identified based on the contrast in theferrite phase identified by XRD is metal Cu can be identified by EDS.

The area of Cu precipitated in the identified ferrite phase isdetermined by image analysis. The sum of the areas of Cu precipitated inthe ferrite phase is divided by the sum of areas of the ferrite phase.The Cu area fraction (%) in the ferrite phase is thus measured.

The duplex stainless steel according to the present embodiment satisfiesboth the chemical composition including Formula (1) and themicrostructure including the in-ferrite-phase Cu area fraction describedabove. The duplex stainless steel according to the present embodimenttherefore has excellent pitting resistance.

[Yield Strength]

The yield strength of the duplex stainless steel according to thepresent embodiment is not limited to a specific value. When the yieldstrength is 750 MPa or less, however, the cold working can be omitted inthe production process. In this case, the production cost can bereduced. The yield strength is therefore preferably 750 MPa or less. Theyield strength is more preferably 720 MPa or less. The lower limit ofthe yield strength is not limited to a specific value and is, forexample, 300 MPa.

[Method for Measuring Yield Strength]

The yield strength in the present specification means 0.2% proof stressdetermined by a method compliant with JIS Z2241 (2011).

[Shape of Duplex Stainless Steel]

The shape of the duplex stainless steel according to the presentembodiment is not limited to a specific shape. The duplex stainlesssteel may be used in a form of, for example, a steel pipe, a steelplate, a steel bar, or a wire rod.

[Production Method]

The duplex stainless steel according to the present embodiment can beproduced, for example, by the following method: The production methodincludes a preparation step, a hot working step, a cooling step, and asolution heat treatment step.

[Preparation Step]

In the preparation step, a starting material having the chemicalcomposition described above is prepared. The starting material may be acast piece produced by a continuous casting process (including roundcontinuous casting) or a slab produced from the cast piece. The startingmaterial may be a slab produced by performing hot working on an ingotproduced by an ingot-making process.

[Hot Working Step]

The prepared starting material is placed in a heating furnace or asoaking pit and heated at a temperature ranging, for example, from 1150to 1300° C. The heated starting material is subsequently subjected tohot working. The hot working may be hot forging, hot extrusion using,for example, the Ugine-Sejournet process or the Ehrhardt push benchprocess, or hot rolling. The hot working may be performed once ormultiple times.

The heated starting material is subjected to hot working at 850° C. ormore. More specifically, the surface temperature of the steel materialat the end of the hot working is 850° C. or more. When the surfacetemperature of the steel material at the end of the hot working is lessthan 850° C., a large amount of Cu precipitates in the ferrite phase. Asa result, even a solution treatment, which will be described later,cannot sufficiently reduce the Cu area fraction in the ferrite phase insome cases. In this case, the pitting resistance of the duplex stainlesssteel lowers. The surface temperature of the steel material at the endof the hot working is therefore 850° C. or more. In a case where the hotworking is performed multiple times, the surface temperature of thesteel material at the end of the last hot working is 850° C. or more.Precipitation of Cu in the ferrite phase can thus be suppressed at theend of the hot working. The upper limit of the surface temperature ofthe steel material at the end of the hot working is not limited to aspecific value and is, for example, 1300° C. The end of the hot workingis the point of time within three seconds after the hot working ends.

[Cooling Step]

The starting material after the hot working is subsequently cooled at arate of 5° C./sec or more. Cu starts precipitating in the ferrite phaseat around 850° C. Therefore, if the cooling rate after the hot workingis too slow, a large amount of Cu precipitates in the ferrite phase. Asa result, even a solution treatment, which will be described later,cannot sufficiently reduce the Cu area fraction in the ferrite phase insome cases. In this case, the pitting resistance of the duplex stainlesssteel lowers. The cooling rate after the hot working is therefore 5°C./sec or more. In the case where the hot working is performed multipletimes, “after the hot working” refers to “after the last hot working.”That is, in the present embodiment, the starting material after the lasthot working is cooled at the rate of 5° C./sec or more. The upper limitof the cooling rate is not limited to a specific value. The coolingmethod is, for example, air cooling, water cooling, or oil cooling.

[Solution Heat Treatment Step]

The cooled starting material is subsequently subjected to a solutionheat treatment at 1070° C. or more. The solution heat treatment causesthe Cu precipitated in the ferrite phase to dissolve. Performing thesolution heat treatment at 1070° C. or more on the starting material inwhich the precipitation of Cu in the ferrite phase at the end of the hotworking and after the cooling is sufficiently suppressed allows the Cuarea fraction in the ferrite phase to be 0.5% or less. The upper limitof the solution heat treatment temperature is not limited to a specificvalue and is, for example, 1150° C. The treatment period of the solutionheat treatment is not limited to a specific value. The treatment periodof the solution heat treatment ranges, for example, from 1 to 30minutes.

The duplex stainless steel according to the present embodiment can beproduced by carrying out the steps described above. In the presentembodiment, it is preferable to perform no cold working because coldworking increases the production cost.

Examples

Alloys having the chemical compositions shown in Table 3 were melted ina 50 kg vacuum furnace, the obtained ingots were heated at 1200° C., andthe heated ingots were subjected to hot forging and hot rolling intosteel plates having a thickness of 10 mm. The temperatures at the end ofrolling shown in Table 4 are the surface temperatures of the steelplates at the end of the hot rolling. The post-rolling cooling ratesshown in Table 4 are the cooling rates after the hot rolling. Further,the steel plates were subjected to a solution treatment at the solutiontemperatures (° C.) shown in Table 4 into test specimens labeled withthe test numbers.

TABLE 3 Chemical composition (unit is mass %, balance is Fe andimpurities) Steel Cr Mo Ni W Cu N C Si Mn sol. Al V O P S Ca Mg B F1 A27.14 3.21 6.21 4.10 0.50 0.406 0.015 0.50 0.98 0.017 0.10 0.003 0.0190.001 0.0019 0.0021 0.0017 59.8 B 28.10 3.11 5.31 4.19 0.14 0.421 0.0160.49 0.97 0.013 0.10 0.004 0.018 0.001 0.0025 0.0001 0.0019 67.2 C 28.242.96 5.76 4.25 0.08 0.416 0.014 0.51 0.91 0.012 0.10 0.004 0.019 0.0010.0015 0.0002 0.0012 69.5 D 27.01 2.50 5.29 4.00 0.09 0.401 0.017 0.520.92 0.014 0.10 0.005 0.017 0.001 0.0027 0.0034 0.0015 65.1 E 27.53 2.616.97 4.31 0.04 0.419 0.016 0.48 0.92 0.017 0.10 0.005 0.016 0.001 0.00100.0025 0.0013 71.1 F 27.88 3.05 5.34 5.61 0.07 0.501 0.016 0.49 0.940.015 0.11 0.003 0.017 0.001 — — — 74.6 G 28.71 3.45 7.21 4.37 0.080.457 0.014 0.48 0.97 0.016 0.11 0.005 0.017 0.001 — — — 73.0 H 27.302.86 6.48 3.61 0.08 0.401 0.018 0.54 0.91 0.014 0.10 0.004 0.021 0.0010.0018 0.0019 0.0021 66.6 I 27.04 2.23 7.62 4.19 0.07 0.405 0.016 0.510.92 0.019 0.11 0.003 0.023 0.001 0.0025 0.0014 0.0011 65.7 J 26.10 3.015.67 4.27 0.09 0.408 0.019 0.47 0.96 0.017 0.09 0.003 0.018 0.001 0.00130.0034 0.0017 66.9

TABLE 4 Analysis results Production conditions Cu area End of fractionPitting rolling Post-rolling Solution Ferrite in ferrite potential YieldTest temperature cooling rate temperature fraction phase Vc′₁₀₀ strengthNo. Steel (° C.) (° c./sec) (° C.) (volume %) (%) (mVvs · SCE) (MPa) 1 A980 30 1120 44 0.8 −60 712 2 B 970 10 1100 48 0.6 71 680 3 C 1010 301050 39 0.7 −12 620 4 D 930 10 1100 43 0.1 85 719 5 E 950 30 1100 50 0.0346 637 6 C 1000 30 1090 41 0.0 204 675 7 F 1020 10 1070 40 0.0 410 6178 G 1060 10 1090 47 0.0 384 701 9 H 1050 10 1100 51 0.0 70 721 10 I 110030 1090 48 0.1 76 679 11 J 1040 10 1070 45 0.2 81 665 12 C 840 10 107044 1.1 −150 663 13 C 1000 3 1090 51 1.6 −71 714

[Ferrite Fraction Measurement Test]

The ferrite fraction (volume %) of each of the test specimens labeledwith the test numbers was measured by using the method described above.Table 4 shows the results of the measurement. The balance of themicrostructure of each of the test specimens labeled with the testnumbers was the austenite phase.

[In-Ferrite-Phase Cu Area Fraction Measurement Test]

The in-ferrite-phase Cu area fraction (%) of each of the test specimenslabeled with the test numbers was measured by using the method describedabove. Table 4 shows the results of the measurement.

[Pitting Potential Measurement Test]

The pitting potential of each of the test specimens labeled with thetest numbers after the solution treatment was measured. The testspecimens were each first machined into a test specimen having adiameter of 15 mm and a thickness of 2 mm. The obtained test specimenswere each used to measure the pitting potential in 25% NaClaq, at 80° C.The conditions other than the test temperature and the NaClconcentration were compliant with JIS G0577 (2014). Table 4 shows theresults of the measurement of pitting potential Vc′₁₀₀ of the testspecimens labeled with the test numbers.

[Tensile Test]

The 0.2% proof stress of the test specimens labeled with the respectivetest numbers was determined by using a method compliant with JIS Z2241(2011). Table 4 shows the results of the determination.

[Evaluation Results]

Referring to Tables 3 and 4, the test specimens labeled with testnumbers 5 to 8 had appropriate chemical compositions and were producedunder appropriate conditions. The test specimens labeled with the testnumbers 5 to 8 therefore were the duplex stainless steel having aferrite fraction ranging from 35 to 65 volume % with the balance beingthe austenite phase, and the Cu area fraction in the ferrite phase was0.5% or less. As a result, the pitting potential (mVvs.SCE) of each ofthe steel plates labeled with the test numbers 5 to 8 was 100 or more,which represented excellent pitting resistance.

On the other hand, the test specimen labeled with test number 1 has toohigh a Cu content. Further, F1 of the test specimen labeled with thetest number 1 was 59.8, which did not satisfy Formula (1). The Cu areafraction in the ferrite phase of the test specimen labeled with the testnumber 1 was therefore 0.8%. As a result, the pitting potential(mVvs.SCE) of the test specimen labeled with the test number 1 was −60,which did not represent excellent pitting resistance.

The test specimen labeled with test number 2 has too high a Cu content.The Cu area fraction in the ferrite phase of the test specimen labeledwith the test number 2 was therefore 0.6%. As a result, the pittingpotential (mVvs.SCE) of the test specimen labeled with the test number 2was 71, which did not represent excellent pitting resistance.

The solution temperature of the test specimen labeled with test number 3was 1050° C. which was too low. The Cu area fraction in the ferritephase of the test specimen labeled with the test number 3 was therefore0.7%. As a result, the pitting potential (mVvs.SCE) of the test specimenlabeled with the test number 3 was −12, which did not representexcellent pitting resistance.

The content of each element of the test specimen labeled with testnumber 4 was appropriate, but F1 was 65.1, which did not satisfy Formula(1). As a result, the pitting potential (mVvs.SCE) of the test specimenlabeled with the test number 4 was 85, which did not represent excellentpitting resistance.

The test specimen labeled with test number 9 had too low a W content. Asa result, the pitting potential (mVvs.SCE) of the test specimen labeledwith the test number 9 was 70, which did not represent excellent pittingresistance.

The test specimen labeled with test number 10 had too low a Mo content.As a result, the pitting potential (mVvs.SCE) of the test specimenlabeled with the test number 10 was 76, which did not representexcellent pitting resistance.

The test specimen labeled with test number 11 had too low a Cr content.As a result, the pitting potential (mVvs.SCE) of the test specimenlabeled with the test number 11 was 81, which did not representexcellent pitting resistance.

The temperature of the test specimen labeled with test number 12 at theend of the hot rolling was 840° C., which was too low. The Cu areafraction in the ferrite phase of the test specimen labeled with the testnumber 12 was therefore 1.1%. As a result, the pitting potential(mVvs.SCE) of the test specimen labeled with the test number 12 was−150, which did not represent excellent pitting resistance.

The cooling rate at which the test specimen labeled with test number 13was cooled at the end of the hot rolling was 3° C./sec, which was tooslow. The Cu area fraction in the ferrite phase of the test specimenlabeled with the test number 13 was therefore 1.6%. As a result, thepitting potential (mVvs.SCE) of the test specimen labeled with the testnumber 13 was −71, which did not represent excellent pitting resistance.

The embodiment of the present invention has been described. Theembodiment described above is, however, only an example for implementingthe present invention. The present invention is therefore not limited tothe embodiment described above, and the embodiment described above canbe changed as appropriate to the extent that the change does not departfrom the substance of the present invention.

1. A duplex stainless steel comprising: a chemical compositionconsisting of in mass %, Cr: more than 27.00% to 29.00%, Mo: 2.50 to3.50%, Ni: 5.00 to 8.00%, W: 4.00 to 6.00%, Cu: 0.01 to less than 0.10%,N: more than 0.400% to 0.600%, C: 0.030% or less, Si: 1.00% or less, Mn:1.00% or less, sol.Al: 0.040% or less, V: 0.50% or less, O: 0.010% orless, P: 0.030% or less, S: 0.020% or less, Ca: 0 to 0.0040%, Mg: 0 to0.0040%, B: 0 to 0.0040%, and with the balance being Fe and impurities,and satisfying Formula (1), a microstructure consisting of 35 to 65volume % of ferrite phase with the balance being an austenite phase,wherein an area fraction of Cu precipitates in the ferrite phase is 0.5%or less:Cr+4.0×Mo+2.0×W+20×N−5×ln(Cu)≥65.2  (1) where, a content in mass % ofeach of the elements is substituted into a corresponding symbol of theelement in Formula (1).
 2. The duplex stainless steel according to claim1, wherein the chemical composition contains, in mass %, one or moretypes of element selected from the group consisting of: Ca: 0.0001 to0.0040%, Mg: 0.0001 to 0.0040%, and B: 0.0001 to 0.0040%.
 3. A methodfor producing a duplex stainless steel, the method comprising the stepsof: preparing a starting material having a chemical compositionconsisting of, in mass %, Cr: more than 27.00% to 29.00%, Mo: 2.50 to3.50%, Ni: 5.00 to 8.00%, W: 4.00 to 6.00%, Cu: 0.01 to less than 0.10%,N: more than 0.400% to 0.600%, C: 0.030% or less, Si: 1.00% or less, Mn:1.00% or less, sol.Al: 0.040% or less, V: 0.50% or less, O: 0.010% orless, P: 0.030% or less, S: 0.020% or less, Ca: 0 to 0.0040%, Mg: 0 to0.0040%, and B: 0 to 0.0040% with the balance being Fe and impuritiesand satisfying Formula (1): subjecting the starting material to hotworking at 850° C. or more; cooling the starting material subjected tothe hot working at a rate of 5° C./sec or more; and subjecting thecooled starting material to a solution heat treatment at 1070° C. ormore:Cr+4.0×Mo+2.0×W+20×N−5×ln(Cu)≥65.2  (1) where, a content in mass % ofeach of the elements is substituted into a corresponding symbol of theelement in Formula (1).